O. S. Ogedengbe

Radiative and interfacial recombination in CdTe heterostructures

Double heterostructures (DH) were produced consisting of a CdTe film between two wide band gap barriers of CdMgTe alloy. A combined method was developed to quantify radiative and non-radiative recombination rates by examining the dependence of photoluminescence (PL) on both excitation intensity and time. The measured PL characteristics, and the interface state density extracted by modeling, indicate that the radiative efficiency of CdMgTe/CdTe DHs is comparable to that of AlGaAs/GaAs DHs, with interface state densities in the low 1010 cm−2 and carrier lifetimes as long as 240 ns. The radiative recombination coefficient of CdTe is found to be near 10−10 cm3s−1. CdTe film growth on bulk CdTe substrates resulted in a homoepitaxial interface layer with a high non-radiative recombination rate.

Impact of extended defects on recombination in CdTe heterostructures grown by molecular beam epitaxy

Heterostructures with CdTe and CdTe1-xSex (x ∼ 0.01) absorbers between two wider-band-gap Cd1-xMgxTe barriers (x ∼ 0.25–0.3) were grown by molecular beam epitaxy to study carrier generation and recombination in bulk materials with passivated interfaces. Using a combination of confocal photoluminescence (PL), time-resolved PL, and low-temperature PL emission spectroscopy, two extended defect types were identified and the impact of these defects on charge-carrier recombination was analyzed. The dominant defects identified by confocal PL were dislocations in samples grown on (211)B CdTe substrates and crystallographic twinning-related defects in samples on (100)-oriented InSb substrates. Low-temperature PL shows that twin-related defects have a zero-phonon energy of 1.460 eV and a Huang-Rhys factor of 1.50, while dislocation-dominated samples have a 1.473-eV zero-phonon energy and a Huang-Rhys factor of 1.22. The charge carrier diffusion length near both types of defects is ∼6 μm, suggesting that recombinati...

Factors influencing photoluminescence and photocarrier lifetime in CdSeTe/CdMgTe double heterostructures

CdSeTe/CdMgTe double heterostructures were produced with both n-type and unintentionally doped absorber layers. Measurements of the dependence of photoluminescence intensity on excitation intensity were carried out, as well as measurements of time-resolved photoluminescence decay after an excitation pulse. It was found that decay times under very low photon injection conditions are dominated by a non-radiative Shockley-Read-Hall process described using a recombination center with an asymmetric capture cross section, where the cross section for holes is larger than that for electrons. As a result of the asymmetry, the center effectively extends photoluminescence decay by a hole trapping phenomenon. A reduction in electron capture cross section appeared at doping densities over 1016cm−3. An analysis of the excitation intensity dependence of room temperature photoluminescence revealed a strong relationship with doping concentration. This allows estimates of the carrier concentration to be made through a non-...

Effect of free-carrier concentration and optical injection on carrier lifetimes in undoped and iodine doped CdMgTe/CdSeTe double heterostructures grown by molecular beam epitaxy

Time-resolved and time integrated photoluminescence (PL) studies are reported for undoped and doped CdMgTe/CdSeTe double heterostructures (DHs) grown by molecular beam epitaxy. Undoped DHs are studied with absorber layer thickness varying from 0.5 to 2.5 µm. The n-type free-carrier concentration is varied ~7 × 1015, 8.4 × 1016, and 8.4 × 1017 cm−3 using iodine as a dopant in different absorber layer thicknesses (0.25–2.0 µm). Optical injection is varied from 1 × 1010 to 3 × 1011 photons/pulse/cm2, corresponding to the initial injection of photo-carriers up to ~8 × 1015 cm−3, to examine the effects of excess carrier concentration on the PL lifetimes. Undoped DHs exhibit an initial rapid decay followed by a slower dependence with carrier lifetimes up to ~485 ns. The dependence of carrier lifetimes on the thickness of the absorber layers (0.5–2.5 µm) suggests interface recombination velocities () ~ 1288 and 238 cm s−1 in the initial and later decay times, respectively, corresponding to high and low photo-carrier concentrations. The Shockley–Read–Hall model is used to describe the results in which variations are observed in for undoped DHs. The lifetimes of doped DHs show a consistent trend with thickness. The ~ 80–200 cm s−1 is estimated for doping n ~ 7 × 1015 cm−3 and 240–410 cm s−1 for n ~ 8.4 × 1016 cm−3. The observed decrease in carrier lifetimes with increasing n is consistent with growing importance of the radiative recombination rate due to the excess carrier concentration. The effect of carrier concentration on the PL spectrum is also discussed.

Determining and Controlling the Magnesium Composition in CdTe/CdMgTe Heterostructures

The relationships between Mg composition, band gap, and lattice characteristics are investigated for Cd1−xMgxTe barrier layers using a combination of cathodoluminescence, energy dispersive x-ray spectroscopy, variable angle spectral ellipsometry, and atom probe tomography. The use of a simplified, yet accurate, variable angle spectral ellipsometry analysis is shown to be appropriate for fast determination of composition in thin Cd1−xMgxTe layers. The validity of using high-resolution x-ray diffraction for CdTe/Cd1−xMgxTe double heterostructures is discussed. The stability of CdTe/Cd1−xMgxTe heterostructures are investigated with respect to thermal processing.

Substrate preparation effects on defect density in molecular beam epitaxial growth of CdTe on CdTe (100) and (211)B

Recent studies have demonstrated that growth of CdTe on CdTe (100) and (211)B substrates via molecular beam epitaxy (MBE) results in planar defect densities 2 and 3 orders of magnitude higher than growth on InSb (100) substrates, respectively. To understand this shortcoming, MBE growth on CdTe substrates with a variety of substrate preparation methods is studied by scanning electron microscopy, secondary ion mass spectrometry, x-ray photoelectron spectroscopy, cross sectional transmission electron microscopy, and atom probe tomography (APT). Prior to growth, carbon is shown to remain on substrate surfaces even after atomic hydrogen cleaning. APT revealed that following the growth of films, trace amounts of carbon remained at the substrate/film interface. This residual carbon may lead to structural degradation, which was determined as the main cause of higher defect density.

Photoluminescence of Crystalline cDtE double heterostructures grown by MBE

Low-temperature photoluminescence (LTPL) and time-resolved photoluminescence (TRPL) were used to study bulk material and interface properties of MBE-grown CdTe. CdTe and CdTe ternary-alloy double heterostructures (DH) grown on CdTe and InSb substrates show LTPL emission from excitons, dislocations, and other defects. Photoluminescence spectra changed with material composition and quality, with near-band exciton emissions increasing and emissions related to extended and point defects decreasing as defect density decreased and interfaces improved. Measured lifetimes from TRPL decay curves also reflected the quality of the DHs. Data showed that overall DH quality depends more upon buffer thicknesses than absorber layer thicknesses. CdTe/CdMgTe DHs grown on InSb substrates had the highest near-band PL and lowest defect emission as seen in low-temperature spectral emission and highest lifetimes in TRPL data.

Study of recombination in CdMgTe/CdTeSe heterostructures using photoluminescence intensity and lifetime measurements with confocal photoluminescence microscopy

Intensity-resolved and time-resolved PL are shown to be powerful tools for analyzing recombination in epitaxial CdTe appropriate for photovoltaic applications. Non-radiative defects such as dislocations are easily mapped and quantified by confocal photoluminescence. Very low dislocation density and twin content, as well as very high luminescence efficiency and measured lifetime (450 ns), can be achieved by Se-alloying to lattice match CdTeSe to InSb substrates. Analysis suggests the bulk lifetime for epitaxial CdTeSe is in excess of 700 ns.